Messages in this thread | | | Date | Fri, 22 Jul 2022 16:15:49 +0800 | Subject | Re: [PATCH] blk-mq: avoid potential infinite loop in __blk_mq_alloc_request | From | Liu Song <> |
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> On Fri, Jul 22, 2022 at 01:22:23PM +0800, Liu Song wrote: >> From: Liu Song <liusong@linux.alibaba.com> >> >> If "blk_mq_get_tag" returns BLK_MQ_NO_TAG because the value of >> "tags->nr_reserved_tags" is 0, it will fall into an infinite loop in >> "__blk_mq_alloc_requests", so borrow BLK_MQ_REQ_NOWAIT to exit the loop. > That means the driver calling blk_mq_alloc_request has a bug, and > we should not work round that in the low level tag allocation path. > > If we want to be nice we can add a WARN_ON before going all the way > down into the tag allocator, something like: > > diff --git a/block/blk-mq.c b/block/blk-mq.c > index 92aae03103b74..d6c7e2ece025f 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -520,6 +520,10 @@ struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op, > struct request *rq; > int ret; > > + if (WARN_ON_ONCE((flags & BLK_MQ_REQ_RESERVED) && > + !q->tag_set->reserved_tags)) > + return ERR_PTR(-EINVAL); > +
Hi,
It is a reasonable approach to prevent abnormal alloc from going down, but this is a very rare exception after all, and above modification is checked every alloc request, which seems to be a bit excessive overhead.
After the rare exception occurs and fix it, the impact on the alloc request will be lower.
Thanks
> ret = blk_queue_enter(q, flags); > if (ret) > return ERR_PTR(ret);
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